feat: notification digests via SMTP

Adds a periodic inbox-summary email per opted-in user. SMTP is
configured via appsettings (Smtp section); the digest is built from
existing analytics (health score, top senders, recommendations) and
sent by a new hourly DigestWorker, mirroring the GmailSyncWorker
pattern. Frequency and send hour are configurable (Digest section).

Backend: IEmailSender (SmtpEmailSender, MailKit), IDigestService,
DigestWorker, new User.DigestEnabled/LastDigestSentUtc fields +
migration, SettingsController for the per-user toggle and a
send-now test endpoint, all scoped to the authenticated UserId.

Frontend: SettingsApi + a bell/no-bell toggle button in the topbar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/DigestToggle.jsx

@@ -0,0 +1,28 @@
+import { useEffect, useState } from 'react';
+import { SettingsApi } from '../api/client.js';
+
+export default function DigestToggle() {
+  const [enabled, setEnabled] = useState(null);
+
+  useEffect(() => {
+    SettingsApi.getDigest().then((d) => setEnabled(d.enabled)).catch(() => setEnabled(false));
+  }, []);
+
+  const toggle = async () => {
+    const next = !enabled;
+    setEnabled(next);
+    try { await SettingsApi.setDigest(next); } catch { setEnabled(!next); }
+  };
+
+  if (enabled === null) return null;
+
+  return (
+    <button
+      className="ghost digest-toggle"
+      title={enabled ? 'Weekly digest emails: on' : 'Weekly digest emails: off'}
+      onClick={toggle}
+    >
+      {enabled ? '🔔' : '🔕'}
+    </button>
+  );
+}